1. Field of the Invention
The present invention relates to a mobile communication system, and specifically relates to a mobile communication system, base transceiver station apparatus, and mobile station apparatus, which comprise a function for compensating for a Doppler shift.
2. Description of the Related Art
As is well known, a Doppler shift caused by the Doppler effect occurs in mobile communication systems. That is, as a mobile station moves in a direction, which shortens the distance between the mobile station and a base transceiver station, the signal frequency received by a receiving station (either one of the mobile station or based transceiver station) is higher than the signal frequency transmitted from a transmitting station (the other one of mobile station or base transceiver station). Contrary, as the mobile station moves in a direction, which elongates the distance between the mobile station and a base transceiver station, the signal frequency received by a receiving station is lower than the signal frequency transmitted from a transmitting station. For this effect, the receiving station has to absorb or compensate for the Doppler shift and regenerate a signal.
FIG. 1 is a diagram describing a configuration of a conventional mobile communication system. In this description, an operation and a configuration relating to the Doppler shift are explained.
A base transceiver station 500 comprises an antenna element 501, a separator (circulator) 502, an orthogonal demodulator 503, a Voltage Control Oscillator (VCO) 504, an uplink frequency synthesizer 505, a phase error detector 506, a synchronous detector 507, a demodulator 508, a encoder 509, a frame generator 510, a multiplexer 511, a downlink frequency synthesizer 512 and a orthogonal modulator 513. The base transceiver station 500 transmits data to a mobile station 600 in a prescribed unique wireless frequency using the VOC 504 and the downlink frequency synthesizer 512. The base transceiver station 500 detects a frequency error (a frequency offset) of received wave from the mobile station apparatus 600 using the phase error detector 506, and regenerates data compensating for the error.
The mobile station apparatus 600 comprises an antennal element 601, a separator (circulator) 602, a orthogonal demodulator 603, a synchronous detector 604, a decoder 605, a handover controller 606, a phase error detector 607, an Automatic Frequency Control (AFC) unit 608, a VOC 609, a downlink frequency synthesizer 610, an encoder 611, a frame generator 612, an uplink frequency synthesizer 613, and an orthogonal modulator 614. The AFC unit 608 controls input voltage of the VOC 609 so that the frequency error (frequency offset) of the received wave from the base transceiver station apparatus 500 converges on zero. The mobile station apparatus 600 regenerates the received data and performs data transmission using a clock generated by the VOC 609. That is to say, the mobile station apparatus 600 uses the same clock for both regenerating data and transmitting data.
Here, assume there is an environment in which a mobile station moves at a high speed on a train (particularly in a train such as a bullet train traveling at a high speed) or in a car, and a base transceiver station is located in a place where the radio path between the mobile station and the base transceiver station along a traveling route of the train or the car is under a line of sight (LOS) condition (a place where direct waves are transmitted and received between the mobile station and the base transceiver station). Under such an environment, the frequency control in a conventional mobile communication system is performed as shown in FIG. 2. In FIG. 2, uplink/downlink frequencies are not distinguished but are described as “fc” in order to simplify the explanation; however, a problem described below also occurs even if the uplink and downlink frequencies are different from each other.
In a path under the LOS condition, because a mobile station and a base transceiver station individually receive direct waves, the Doppler shift equals to the frequency offset. Therefore, the mobile station has to perform AFC, which keeps up with wireless frequency including the Doppler shift.
In FIG. 2, when a mobile station (MS) approaches to a base transceiver station (BTS1), the frequency of received wave on the antenna in the MS is higher by the amount of the Doppler shift. Therefore, as a result of AFC on the received wave, the frequency (downlink frequency) of a periodic wave used in the mobile station to receive a signal is controlled by “fc+fd”. Here, “fc” is a reference frequency of a carrier wave, and “fd” is a Doppler shift frequency. When an ideal AFC is performed, the frequency offset that the mobile station receives after an orthogonal modulation becomes zero.
In this example, as explained with reference to FIG. 1, the same frequency as the downlink frequency obtained from the AFC is used as a frequency of a carrier wave for data transmission (uplink frequency) in the conventional mobile communication system. Therefore, the uplink frequency is also controlled at “fc+fd”. When a radio wave with the frequency “fc+fd” is transmitted from the mobile station, the Doppler shift is also added to the uplink as well, and consequently, the frequency of the received wave in the base transceiver station (BTS1) becomes “fc+2fd”. In other words, the frequency offset after the orthogonal modulation in the base transceiver station is twofold of the Doppler shift.
When the mobile station (MS) passes in proximity of the base transceiver station (BTS1), a state in which the mobile station approaches to the base transceiver station changes into a state in which the mobile station recedes from the base transceiver station, and therefore, the polarity of the Doppler shift can be inverted within a short time period. At that time, the fluctuation in the Doppler shift fd can be expressed by the following equation where “v” represents a moving velocity of the mobile station, “c” is speed of light, “x” is a vertical distance from the base transceiver station (BTS1) to the traveling route of the mobile station, “t” is a elapsed time on the basis of the time when the mobile station passes in proximity of the base transceiver station (BTS1), and “θ(t)” is an elevation angle when the base transceiver station (BTS1) is seen from the traveling direction of the mobile station.
  fd  =                    fc        ⨯                  v          c                    ⁢      cos      ⁢                          ⁢              θ        ⁡                  (          t          )                      =          fc      ⨯              v        c            ⨯                                  vt                                                              x              2                        +                                          v                2                            ⁢                              t                2                                                        
FIG. 3 is diagram showing fluctuation in the Doppler shift obtained by the above equation. As the moving velocity of the mobile station increases, the fluctuation range of the Doppler shift becomes wider. As the vertical distance from the base transceiver station to the moving route of the mobile station is smaller, the Doppler shift fluctuates within a shorter time period.
For example, when fc=2 GHz, v=300 km/h, and x=50 m, the received frequency of the mobile station drastically fluctuates from “2 G+600” Hz to “2 G−600” Hz within a several seconds, and therefore, in order to minimize the frequency offset in the mobile station, the time constant of the AFC has to be set at high speed. However, if the time constant of the AFC is set at high speed, the average time of the phase error detection is short with respect to noise and control steps of the frequency control is inexact. That is to say, increase in speed of the AFC and the accuracy of the frequency control are in a tradeoff relation.
In order to solve this problem, Patent Document 1 (Japanese Patent Application Laid-open Publication No. 2002-101012) for example suggests a technology to make the AFC band and tracking speed variable in accordance with the communication conditions. However, by introducing this configuration, another problem rises that the AFC circuit in a mobile station becomes more complicated.
In addition, even if the AFC in the mobile station is performed as desired, the base transceiver station receives a frequency offset equivalent to twofold of the Doppler shift. For that reason, in the above case, the received frequency in the base transceiver station fluctuates by 2400 Hz from “2 G+1200” Hz to “2 G−1200” Hz within a several seconds. Consequently, the requirement of increase in the speed in the frequency compensation in the base transceiver station (frequency compensation processing in synchronous detection based on the result of phase error detection) is further difficult than the conditions in the mobile station.
However, in general, the base transceiver station is designed to compensate for a frequency error of only 0.1 ppm caused by the frequency stability of the mobile station. Here, when fc=2 GHz, the frequency error that can be compensated is as much as ±200 Hz. Therefore, in order to secure the reception quality under specific conditions that is an environment of high-speed moving in a path under LOS condition, the base transceiver station must be implemented with a frequency compensation circuit with the compensation range six times as wide as a usual design. Additionally, in order to realize the high-speed tracking, the frequency compensation accuracy has to be sacrificed.
In FIG. 2, assume a handover caused when a mobile station (MS) moves from a communication area of a base transceiver station (BTS1) to a communication area of another base transceiver station (BTS2). In this description, the assumed handover is a soft handover (SHO) in CDMA employed by the second generation and the third generation cellular phone systems. In the CDMA, the same frequency can be used in cells adjacent to each other. Therefore, the mobile station can switch reference cells, synthesizing signals from a plurality of base transceiver stations at the maximum ratio (i.e. soft handover).
However, a conventional mobile station (MS) can perform the AFC on only one received frequency. For that reason, the mobile station generally performs the AFC on the received frequency of a reference cell alone. Then, as shown in FIG. 2, uplink/downlink frequency is controlled at “fc−fd” while the base transceiver station (BTS1) is the reference cell, and the uplink/downlink frequency is controlled at “fc+fd” after the reference cell is changed from the base transceiver station (BTS1) to the base transceiver station (BTS2). In other words, the mobile station synthesizes a pair of received waves with a frequency difference equivalent to twofold of the Doppler shift at the maximum ratio in the handover area. As a result, in addition that diversity gain by the soft handover cannot be obtained, it is possible that the reception quality is more deteriorated than the reception quality when the synthesize is not performed.
Another mobile communication system regarding to the Doppler shift is described in Patent Document 2 (Japanese Patent Application Laid-open Publication No. H10-200471), for example. A technology relating to the AFC in handover is described in Patent Document 3 (Japanese Patent Application Laid-open Publication No. H11-355826), for example.